1. Field of the Invention
The present invention relates to a radiographic intensifying screen and, more particularly, to a radiographic intensifying screen comprising a support, a phosphor layer which comprise a binder and a phosphor dispersed therein and a light-reflecting layer containing a white pigment provided between the support and the phosphor layer.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondistructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film for enhancing the radiographic speed of the system. The radiographic intensifying screen consists essentially of a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shocks.
The phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays having passing through an object, the phosphor particles emit light of high luminance (spontaneous emission) in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a relatively small dose.
It is desired for a radiographic intensifying screen to exhibit a high radiographic speed and to provide an image of high quality (i.e., high sharpness, high graininess, etc.). There has been conventionally proposed a variety of intensifying screens improved in the radiographic speed or the quality of the image provided thereby.
For enhancing the radiographic speed of a radiographic intensifying screen, it has been known that a light-reflecting layer is provided between the support and the phosphor layer, for instance, by coating a dispersion comprising a binder and a white pigment on the support to form a light-reflecting layer and subsequently forming the phosphor layer on the light-reflecting layer. Examples of the white pigment employable in the light-reflecting layer have included titanium dioxide, white lead, zinc sulfide, aluminum oxide and magnesium dioxide.
For instance, when a phosphor which emits light in the near ultraviolet region as well as the visible region, such as a divalent europium activated alkaline earth metal fluorohalide phosphor (which has an emission intensity in the near ultraviolet region higher than that in the visible region) is employed for the radiographic intensifying screen, the light-reflecting layer containing one of the above-mentioned white pigments other than magnesium oxide does not show sufficiently high reflection characteristics and the radiographic speed of the intensifying screen is not enhanced to a satisfactory level. That is because such white pigments other than magnesium oxide show considerably low reflectance in the near ultraviolet region, though which show high reflectance in the visible region (in other words, they do not show the reflection spectra in the near ultraviolet region).
Among the above-mentioned white pigments, titanium dioxide is industrially prepared by the sulfate process (Norway Method) or the chloride process, while magnesium oxide is industrially prepared by calcining magnesium carbonate or magnesium hydroxide. Thus prepared white pigments are in the form of particles having small size, usually not more than 1 .mu.m. A pigment having such a small particle size is poor in dispersibility in a binder solution for the formation of light-reflecting layer, and the surface of the resulting light-reflecting layer tends to show poor smoothness, owing to the aggregation of the pigment particles on the surface of the layer. Such a light-reflecting layer having poor smoothness brings about difficulty in the formation of a phosphor layer with an even thickness thereon.